1. Field of the Invention
The invention generally relates to a rod loading fixture which is particularly useful in a system for remotely repairing nuclear fuel rod assemblies having damaged fuel rods or damaged skeletons.
2. Description of the Prior Art
Tools for loading rods into nuclear fuel rod assemblies are known in the prior art. Such tools may be used to reassemble a new fuel rod assembly from an assembly having a damaged support skeleton. In reassembly-type repairs, all of the undamaged fuel rods are removed from the damaged support skeleton and inserted into a new and empty skeleton. In order to implement the loading of the salvaged rods into the new skeleton, manual tools have been developed for individually gripping, lifting, lowering and ungripping specific fuel rods from the damaged support skeleton to the new skeleton.
Unfortunately, the use of such prior art manual tools is not without shortcomings. However, before these shortcomings may be fully appreciated, some brief background as to the structure, operation, and environment of such fuel rod assemblies is necessary.
Nuclear fuel rod assemblies generally comprise 200 to 290 fuel rods mounted in a square array within a support skeleton. The support skeleton in turn is formed from bottom and top nozzle assemblies which are connected to one another by 24 uniformly arrayed thimble rods. The bottom and top nozzles are about 8 to 9 inches square, and the thimble rods are about 13 feet long, so that the overall shape of the fuel assembly is that of an elongated, rectangular prism (see FIG. 17). The fuel rods themselves are about 12 feet long. In order to equidistantly space the long and relatively flimsy fuel rods within the support skeleton, the skeleton includes approximately seven grids, each of which has a square array of apertures for receiving and spacing the fuel rods. The grids are usually sheet-metal structures fabricated from a heat-treated, high strength stainless steel in an "egg crate" type of design which lends compressive strength to the grids with a minimum of weight. In operation, an array of nuclear fuel rod assemblies is placed in the reactor core, and a jet of pressurized water is guided through the bottom nozzles thereof in order to uniformly absorb the heat generated by the rods. In nuclear reactors of the type designed by Westinghouse Electric Corporation, the assignee of the present invention, the velocity of the pressurized water forced through the bottom nozzles of the fuel structures is on the order of 15 ft./sec.
In some nuclear cores, this 15-ft./sec. flow of water has created pressure differentials which in turn have resulted in side-currents which flow laterally through the fuel rod assemblies disposed in the core. These side currents sometimes produce vibrations in the fuel rods which can eventually weaken and break the rods through a fretting action, as well as damage the support skeletons themselves.
In order to reassemble new fuel rod assemblies from the damaged ones, the damaged assembly is typically lowered into the cask-loading area (or shaft) of the spent fuel area of the nuclear plant. The cask-loading shaft is approximately 40 feet long, and filled with water in order to shield workers (who typically stand on a deck located over the shaft) from radiation. In reassembling new fuel rod assemblies, the workers on the deck over the cask-loading shafts use elongated hand tools capable of gripping and withdrawing a single rod out of the damaged fuel assembly after the top nozzle has been cut and removed therefrom. Small television cameras are often mounted on these tools so that the workers may visually position them over a particular fuel rod. While such tools are capable of gripping, lifting, lowering and ungripping either damaged or undamaged fuel rods within a support skeleton, they are also long and flimsy, and hence slow and cumbersome to use. Additionally, when inserting a new or salvaged fuel rod into a new skeleton, it is sometimes difficult to "thread" the fuel rod through a set of mutually-aligned, rod-receiving apertures in the grids. If the rod should become slightly tilted during the insertion process, it may be erroneously threaded through a set of misaligned apertures. This in turn can cause the rod to become bent and stuck in an improper position in the grids. Further, while the water in the cask-loading shaft does afford an effective shield for the majority of radiation emanating from the fuel rod assembly being repaired, the substantial amount of time it takes to reload rods into a new skeleton causes workers positioned on the deck to receive some dosage of potentially hazardous radiation.
Clearly, there is a need for a device which allows fuel rods to be loaded rapidly and reliably in order to minimize the amount of radiation to which the workers are exposed, as well as to reduce the chances of a fuel rod becoming bent or damaged during the loading procedure.